The ability of rat liver and intestine to synthesize the individual constituent apoproteins of plasma lipoproteins was determined. A different pattern of synthesis was observed for each organ, studied individually by perfusion techniques under physiological conditions. Isolated, perfused livers synthesized mostly the arginine-rich protein (ARP), apolipoprotein B (apoB) and the apoC peptides. To study intestinal synthesis, an in situ jejunal segment preparation with intact arterial supply and totally collected lymph and venous drainage was developed. Intestine synthesized, in addition to apoB, mostly apoA-I and apoA-IV and almost no ARP and apoC. Most, but not all, of the intestinal products reached the blood by way of mesenteric lymph. Double-label experiments, in which plasma proteins derived from liver and intestine were differentially labeled in vivo, have confirmed in the intact animal the important role of the small intestine in providing two of the circulating apolipoproteins--apoA-I and apoA-IV. In other studies, also with the auto-perfused intestinal preparation in situ.